1. Field of the Invention
This invention relates to a millimeter wave sensor operating primarily on a single frequency to provide online inspection, preferably including continuous monitoring, of thin sheet dielectric materials, such as fabric, paper, plastic and other dielectric products with respect to macroscopic and/or microscopic properties such as density, homogeneity, for example internal flaws, structure, porosity, moisture content, and state of cure. More particularly, the invention relates to a sensor and instrument system operating primarily on a single frequency within the range of about 75-110 GHz with the frequency selected on the characteristics of the sample to provide greater resolution, sensitivity and other data important for the inspection. Of particular importance is the measurement of the amplitude and phase characteristics of the reflected/transmitted signal as a result of variations in properties of the material medium.
2. Description of the Prior Art
The use of microwaves for nondestructive evaluation (NDE) of materials is gaining importance due to the inherently noncontact nature of the technique, good depth of penetration in low-loss dielectric materials, and the emergence of new and advanced materials that pose new NDE challenges that require new solutions. The material property that is sensed by microwave techniques is the change in dielectric properties, such as permittivity and loss factor.
"MILLIMETER WAVE IMAGING FOR NONDESTRUCTIVE EVALUATION OF MATERIALS", Material Evaluation, Volume 52, Number 3, March, 1994 Pps. 412-415, by Nachappa Gopalsami, Sasan Bakhtiari, Stephen L. Dieckman, Apostolos C. Raptis, and Matthew J. Lepper, describes a millimeter-wave imaging system in the W band (75-110 GHz) for nondestructive evaluation of low-loss materials. The subject matter of the above-identified publication is incorporated herein by reference. A theoretical analysis of the millimeter-wave imaging is provided.
A W-band imaging system was initially developed for high resolution inspection of electrically thick samples of stratified dielectric slabs. The system operated either in a bistatic forward-scatter configuration with separate antennas to transmit and receive or a monostatic backscatter configuration with a single antenna used both to transmit and receive. The system offered minimal diffraction loss due to incorporation of Gaussian optic lens antennas. The antenna lenses had an aperture diameter D=12" and a focal length f=12" (i.e., f/D=1). The spot size of the beam at the focal point was about one wavelength (3-dB beamwidth) and had a depth of focus of about 10.lambda.. The backward-wave oscillator (BWO) source output frequency was stabilized using a source-locking frequency counter. A sample to be tested was mounted in a plane perpendicular to the incident beam axis within the focal depth of the antenna. In the monostatic case, a circulator or a hybrid coupler separated the transmitted and reflected waves. Only the amplitude of the scattered fields or the received power was measured with this apparatus. The output detected via a diode detector was amplified, digitized, and acquired by a personal computer. The computer also controlled a two-axis translation stage for scanning the sample.
A principal object of the present invention is to provide an improved millimeter wave(MMW) sensor for on-line inspection of thin sheet dielectrics.
It is another object of the present invention to provide such an improved millimeter wave (MMW) sensor for on-line inspection of thin sheet dielectrics that operates primarily on a single frequency within the range of about 75 GHz to 110 Ghz, that is compact and has a small size antenna, and that is self calibrating.
It is another object of the present invention to provide such an improved millimeter wave (MMW) sensor for on-line inspection of thin sheet dielectrics that operates primarily on a single frequency within the range of about 75 GHz to 110 GHz with the single frequency selected on the basis of characteristics of a particular sample to provide greater resolution, sensitivity and other data important for the inspection.
It is another object of the present invention to provide such an improved millimeter wave sensor for on-line inspection of thin sheet dielectrics that measures both the amplitude and phase characteristics of the reflected/transmitted signal as a result of variations in properties of the material medium.
It is another object of the present invention to provide such an improved millimeter wave sensor for on-line inspection of thin sheet dielectrics that overcomes many of the disadvantages of prior art arrangements.